Surface Decontamination of Fruits and Vegetables Eaten Raw

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Surface decontamination of fruits and vegetables eaten raw: a review

Introduction

Raw fruits and vegetables have been known to serve as vehicles of human disease for at leasta century. In 1899, Morse linked typhoid infection to eating celery. Warry (1903) attributedan outbreak of typhoid fever to eating watercress grown in soil fertilized with sewage and

Pixley (1913) recorded two cases of typhoid from eating uncooked rhubarb which was grown in soilknown to have been fertilized with typhoid excreta. In 1912, Creel demonstrated that lettuce andradishes grown in soil containing Bacillus typhosa (now Salmonella Typhi) harboured the organismon their surfaces for up to 31 days. Melick (1917) recovered typhoid bacilli from mature lettuce andradish harvested from soil that had been inoculated at the time seeds were planted. Some parasitichelminths (e.g. Fasciola hepatica, Fasciolopsis buski) require encystment on plants to completetheir life cycle. Thus, the recognition of raw fruits and vegetables as potential vehicles fortransmission of pathogenic microorganisms known to cause human disease is not new.Nevertheless, documented outbreaks of foodborne illness associated with fruits and vegetables inindustrialized countries are relatively rare. For instance, in 1996 only six of about 200 reportedfoodborne disease outbreaks in the United Kingdom were associated with consumption of fruitsand vegetables (PHLS, 1996). In recent years, however, the frequency of outbreaksepidemiologically associated with raw fruits and vegetables is documented to have increased insome industrialized countries (e.g the United States) as a result of change in dietary habits andincreased import of food (Altekruse et al., 1997). In developing countries, foodborne illnessescaused by contaminated fruits and vegetables are frequent and in some areas they cause a largeproportion of illness. However, due to lack of foodborne disease investigation and surveillance inmost of these countries, most outbreaks go undetected and the scientific literature reports only onvery few outbreaks. In 1995-1996, only 2% of foodborne disease outbreaks in Latin America wererelated to fruits and vegetables (PAHO/INPPAZ, 1996).

Raw and minimally-processed fruits and vegetables are an essential part of people�s diet allaround the world. Where land is available, families grow fruits and vegetables for their own use.Alternatively, produce is purchased from local farmers or retail outlets for further preparation bystreet vendors, by families at home or as part of meals eaten in restaurants and other food-servicefacilities. While advances in agronomic practices, processing, preservation, distribution andmarketing have enabled the raw fruit and vegetable industry to supply high-quality produce tomany consumers all year round, some of these same practices have also expanded the geographicaldistribution and incidence of human illness associated with an increasing number of pathogenicbacterial, viral and parasitic microorganisms.

Changes that may contribute to the increase in diseases associated with the consumption of rawfruits and vegetables in industrialized countries (Hedberg et al., 1994) and foods in general(Altekruse and Swerdlow, 1996; Altekruse el al., 1997; Potter et al., 1997) have been described.Factors include globalization of the food supply, inadvertent introduction of pathogens into newgeographical areas � e.g. outbreaks of shigellosis in Norway, Sweden and the United Kingdom in1994 due to contaminated lettuce imported from southern Europe (Frost et al., 1995; Kapperud etal., 1995) and cyclosporiasis in the United States which was linked to consumption ofcontaminated raspberries imported from Guatemala (Centers for Disease Control and Prevention,1996c) � and the development of new virulence factors by microorganisms, decreases inimmunity among certain segments of the population, and changes in eating habits. In developingcountries, continued use of untreated wastewater and manure as fertilizers for the production of

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fruits and vegetables is a major contributing factor to contamination that causes numerousfoodborne disease outbreaks.

While every effort should be made to prevent contamination of fruits and vegetables duringproduction, transport, processing and handling, much improvement is still needed in some parts ofthe world if hygienic production of fruits and vegetables is to be ensured. Furthermore, manymicrobial contaminants are part of the environment and fruits and vegetables may be inadvertentlycontaminated. Unless measures are taken to decontaminate them, their safety may not be assured.This document provides an overview of the hazards associated with fruits and vegetables eatenraw, reviews the methods used for their decontamination (particularly with reference to chemicalmethods) and evaluates, on the basis of available scientific data, the efficacy of these methods. Thepurpose of this review is to provide public health authorities with information on the surfacedecontamination of fruits and vegetables eaten raw, and with guidelines and recommendations thatcan be provided to growers, processors and consumers.

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Recognizing the problem

Fruits and vegetables can become contaminated with microorganisms capable of causinghuman diseases while still on the plant in fields or orchards, or during harvesting, transport,processing, distribution and marketing, or in the home. Figure 1 shows potential mechanisms

by which pathogenic microorganisms can contaminate vegetables. Bacteria such as Clostridiumbotulinum, Bacillus cereus and Listeria monocytogenes, all capable of causing illness, are normalinhabitants of many soils, whereas Salmonella, Shigella, Escherichia coli and Campylobacterreside in the intestinal tracts of animals, including humans, and are more likely to contaminate rawfruits and vegetables through contact with faeces, sewage, untreated irrigation water or surfacewater. Contamination may also occur during post-harvest handling, including at points ofpreparation by street vendors, in food-service establishments and in the home. Contamination withviruses or parasites can result from contact with faeces, sewage and irrigation water (Cliver, 1997;Speer, 1997).

Numerous surveys have been carried out in many countries to determine the presence ofpathogenic microorganisms on raw fruits and vegetables. In many instances, bacterial pathogenshave not been detected. In other investigations, high percentages of samples have been found tocontain bacteria capable of causing human disease. Results of these investigations are summarizedin Table 1. This list is not comprehensive but it reflects the scope of potential problems that may linkraw fruits and vegetables to human disease. Indeed, numerous outbreaks of illness caused bybacteria, viruses and parasites have been linked epidemiologically to consumption of raw fruits andvegetables (Table 2). Increased recognition of raw fruits and vegetables as suspected vehicles ofhuman illness in industrialized countries (Altekruse et al., 1997; Bean and Griffin, 1990; Bean et al.,1997; Centers for Disease Control and Prevention, 1996b) may probably result in increasednumbers of associated or confirmed cases in the future. Improved diagnostic systems andsurveillance programmes will enhance the prospect of identifying raw fruits and vegetables assources of foodborne illness, thus possibly resulting in an increase in recorded numbers ofoutbreaks and cases of disease. Surveys of raw fruits and vegetables for the presence of parasites orviruses are few, largely because of the lack of sensitive methods for their detection in plantmaterials.

Other factors, however, may contribute to real increases in diseases associated with fruits andvegetables (Hedberg et al., 1994). These include use of wastewater, increased application ofimproperly composted manures to soils in which fruits and vegetables are grown, changes inpackaging technology such as the use of modified or controlled atmosphere and vacuumpackaging, extended time between harvesting and consumption, and changing food consumptionpatterns (e.g. eating more meals away from home, including greater use of salad bars). Increasedglobal trade in raw fruits and vegetables, as well as increased international travel in general, couldalso increase the risk of produce-associated diseases. Finally, the susceptibility of the public tofoodborne diseases, at least in more developed countries, is changing due to increased numbers ofpeople who are elderly, immunocompromised or have chronic diseases. This change in socialdemographics is likely to lead to increased risk of illness associated with the consumption of rawproduce that otherwise may contain levels of pathogens innocuous to healthy individuals.

Prevention of contamination of fruits and vegetables with pathogenic microorganisms should bethe goal of everyone involved in both the pre-harvest and post-harvest phases of delivering produceto the consumer. This is a very difficult task, since some pathogens are normally present in the soiland may therefore be present on the surface of fruits and vegetables when they are harvested. In

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O/FS

F/FOS

/98.2

ANIMALS water PRODUCE HUMANS

Figure 1. Mechanism

s by which raw

fruits and vegetables may becom

e contaminated w

ithpathogenic m

icroorganisms.

From Beuchat (1996b), and used with permission from the International Association of Milk, Food and Environmental Sanitarians.

faeces

sewage

soil

harvesting, handling,processingenvironment

plants silage meat, milk, eggs feed

insects

(cross contamination)

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Table 1. Bacterial pathogens isolated from raw vegetables1

Vegetable Country Pathogen Prevalence Reference

Alfalfa United States Aeromonas Callister and Agger (1989)Artichoke Spain Salmonella 3/25 (12.0%) Garcia-Villanova Ruiz et al. (1987b)Asparagus United States Aeromonas Berrang et al.Bean sprouts Malaysia L. monocytogenes 6/7 (85%) Arumugaswamy et al. (1994)

Malaysia Salmonella 2/10 (20%) Arumugaswamy et al. (1994)Sweden Salmonella Andersson and Jong (1989)Thailand Salmonella 30/344 (8.7%) Jerngklinchan and Saitanu (1993)

Beet leaves Spain Salmonella 4/52 (7.7%) Garcia-Villanova Ruiz et al. (1987b)Broccoli Canada L. monocytogenes 2/15 (13.3%) Odumeru et al. (1997)

United States Aeromonas Berrang et al. (1989)United States Aeromonas 5/16 (31.3%) Callister and Agger (1989)

Cabbage Canada L. monocytogenes 2/92 (2.2%) Schlech et al. (1983)Canada L. monocytogenes 1/15 (6.7%) Odumeru et al. (1997)Mexico E. coli O157:H7 1/4 (25.0%) Zepeda-Lopez et al. (1995)Peru V. chlolerae Swerdlow et al. (1992)Saudi Arabia L. monocytogenes Salamah (1993)Saudi Arabia Y. enterocolitica Salamah (1993)Spain Salmonella 7/41 (17.1%) Garcia-VillanovaRuiz et al.(1987b)United States C. botulinum 1/337 (0.3%) Lilly et al. (1996)United States L. monocytogenes 1/92 (1.1%) Heisick et al. (1989b)

Carrot Lebanon Staphylococcus (14.3%) Abdelnoor et al. (1983)Saudi Arabia L. monocytogenes Salamah (1993)Saudi Arabia Y. enterocolitica Salamah (1993)

Cauliflower Netherlands Salmonella 1/13 (7.7%) Tamminga et al. (1978)Spain Salmonella 1/23 (4.5%) Garcia-Villonova Ruizetal. (1987b)United States Aeromonas Berrang et al. (1989)

Celery Mexico E. coli O157:H7 6/34 (17.6%) Zepeda-Lopez et al. (1995)Spain Salmonella 2/26 (7.7%) Garcia-Villanova Ruizetal.(1987b)

Chili Surinam Salmonella 5/16 (31.3%) Tamminga et al. (1978)Cilantro Mexico E. coli O157:H7 8/41 (19.5%) Zepeda-Lopez et al. (1995)Coriander Mexico E. coli O157:H7 2/10 (20.0%) Zepeda-Lopez et al. (1995)Cress sprouts United States B. cereus Portnoy et al. (1976)Cucumber Malaysia L. monocytogenes 4/5 (80%) Arumugaswamy et al. (1994)

Pakistan L. monocytogenes 1/15 (6.7%) Vahidy (1992)Saudi Arabia L. monocytogenes Salamah (1993)Saudi Arabia Y. enterocolitica Salamah (1993)United States L. monocytogenes Heisick et al. (1989b)

Egg plant Netherlands Salmonella 2/13 (1.5%) Tamminga et al. (1978)Endive Netherlands Salmonella 2/26 (7.7%) Tamminga et al. (1978)Fennel Italy Salmonella 4/89 (71.9%) Ercolani (1976)Green onion Canada Campylobacter 1/40 (2.5%) Park and Sanders (1992)Leafy vegetables Malaysia Salmonella 1/24 (4%) Arumugaswamy et al. (1995)

Malaysia L. monocytogenes 5/22 (22.7%) Arumugaswamy et al. (1994)Leeks Spain L. monocytogenes 1/5 (20%) de Simon et al. (1992)Lettuce Italy Salmonella 82/120 (68%) Ercolani (1976)

Canada Campylobacter 2/67 (3.1%) Park and Sanders (1992)Canada L. monocytogenes 3/15 (20%) Odumeru et al. (1997)Lebanon Staphylococcus (14.3%) Abdelnoor et al. (1983)Netherlands Salmonella 2/28 (7.1%) Tamminga et al. (1978)Saudi Arabia L. monocytogenes Salamah (1993)Saudi Arabia Y. enterocolitica Salamah (1993)Spain Salmonella Garcia-Villanova Ruizetal.(1987b)United States Aeromonas Callister and Agger (1989)

Mungbean sprouts United States Salmonella O�Mahony et al. (1990)Mushrooms United States C. jejuni 3/200 (1.5%) Doyle and Schoeni (1986)Mustard cress United Kingdom Salmonella Joce et al. (1990)Mustard sprouts United States B. cereus Portnoy et al. (1976)

Canada Campylobacter 1/42 (2.4%) Park and Sanders (1992)Parsley Egypt Shigella 1/250 (0.4%) Satchell et al. (1990)

Lebanon Staphylococcus (7.7%) Abdelnoor et al. (1983)Spain Salmonella 1/23 (4.3%) Garcia-Villanova Ruizetal.(1987b)

Pepper Canada L. monocytogenes 1/15 (6.7%) Odumeru et al. (1997)Sweden Salmonella Andersson et al. (1989)United States C. botulinum 1/201 (0.5%) Lilly et al. (1996)United States Aeromonas Callister and Agger (1989)

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Vegetale Country Pathogen Prevalence Peference

Potatoes Saudi Arabia L. monocytogenes Salamah (1993)Saudi Arabia Y. enterocolitica Salamah (1993)Spain L. monocytogenes 2/12 (16.7%) de Simon et al. (1992)United States L. monocytogenes 19/70 (27.1%) Heisick et al. (1989a)United States L. monocytogenes 28/132 (21.1%) Heisick et al. (1989b)Canada Campylobacter 1/63 (1.6%) Park and Sanders (1992)

Prepacked salads Northern Ireland L. monocytogenes 3/21 (14.3%) Harvey and Gilmour (1993)United Kingdom L. monocytogenes 4/60 (13.3%) Sizmur and Walker (1988)United Kingdom L. monocytogenes Velani and Roberts (1991)

Radish Lebanon Staphylococcus (6.3%) Abdelnoor et al. (1983)Saudi Arabia L. monocytogenes Salamah (1993)Saudi Arabia Y. enterocolitica Salamah (1993)United States L. monocytogenes 25/68 (36.8%) Heisick et al. (1989a)Canada Campylobacter 2/74 (2.7%) Park and Sanders (1992)United States L. monocytogenes 19/132 (14.4%) Heisick et al. (1989b)

Salad greens Egypt Salmonella 1/250 (0.4%) Satchell et al. (1990)United Kingdom S. aureus 13/256 (5.1%) Houang et al. (1991)

Salad vegetables Canada L. monocytogenes 6/15 (40%) Odumeru et al. (1997)Egypt Shigella 3/250 (1.2%) Satchell et al. (1990)Egypt Saureus 3/36 (8.3%) Satchell et al.(1990)Spain Aeromonas 2/33 (6.1%) Garcia-Gimeno et al. (1996a)Spain L. monocytogenes 21/70 (30%) Garcia-Gimeno et al. (1996b)United States Staphylococcus Harris et al. (1975)Germany L. monocytogenes 6/263 (2.3%) Breer and Baumgartner (1992)Northern Ireland L. monocytogenes 4/16 (25%) Harvey and Gilmour (1993)United States C. botulinum 2/82 (2.4%) Lilly et al. (1996)United Kingdom Y. enterocolitica Brockelhurst et al. (1987)

Seed sprouts Canada Staphylococcus 13/54 (24%) Prokopowich and Blank (1991)Soybean sprouts United States B. cereus Portnoy et al. (1976)Spinach Canada Campylobacter Park and Sanders (1992)

Spain Salmonella 2/60 (3.3%) Garcia-Villanova Ruizetal.(1987b)United States Aeromonas 2/38(5.2%) Callister and Agger (1989)

Sprouting seeds United States B. cereus 56/98 (57%) Harmon et al. (1987)Tomato Pakistan L. monocytogenes 2/15 (13.3%) Vahidy (1992)Vegetables Egypt Salmonella 2/250 (0.8%) Satchell et al. (1990)

France Y. enterocolitica 4/58 (7%) Catteau et al. (1985)France Y. enterocolitica 15/30 (50%) Darbas et al.(1985)Iraq Salmonella 3/43 (7%) Al-Hindawi and Rished (1979)Italy L. monocytogenes 7/102 (6.9%) Gola et al. (1990)Italy Y. enterocolitica 1/102 (1.0%) Gola et al. (1990)Spain L. monocytogenes 8/103 (7.8%) de Simon et al. (1992)Spain Salmonella 46/849 (5.4%) Garcia-Villanova Ruizetal.(1987a)Taiwan L. monocytogenes 6/49 (12.2%) Wong et al. (1990)United Kingdom L. monocytogenes 4/64 (6.2%) MacGowan et al. (1994)United States Salmonella 4/50 (8.0%) Rude et al. (1984)

1 Adapted from Beuchat (1996b), with permission from the International Association of Milk, Food andEnvironmental Sanitarians. Most vegetables were grown in the country in which they were analysed for thepresence of pathogens.

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addition, inadequate water resources may encourage the use of unsafe water to make ice or rawsewage to irrigate fields. Farm environments are not and cannot be aseptic. Reduction in thechances of contamination can be achieved, however, through appropriate agronomic practices,harvesting, processing, shipping, marketing and preparation.

The use of properly composted manure and properly treated irrigation and spray waters, as wellas pathogen-free water for washing, and for ice, will minimize the risk of contamination of fruits andvegetables with microbial pathogens. Good hygienic practice during production and transport,including sanitizing of harvesting equipment and transport vehicles, as well as the application ofgood hygienic practice during processing and preparation are critical. Elimination of animals andinsects from processing, storage, marketing and food-service facilities should be a goal of anyonewho handles raw produce. The highest level of hygiene must be practised by all handlers (includingconsumers) of fruits and vegetables, from the field to the table, if any degree of success is to beachieved in minimizing the risk of contamination.

The microorganisms normally present on the surface of raw fruits and vegetables may consist ofchance contaminants from the soil or dust, or bacteria or fungi that have grown and colonized byutilizing nutrients exuded from plant tissues. Among the groups of bacteria commonly found onplant vegetation are those that test positive for coliforms or faecal coliforms � e.g. Klebsiella andEnterobacter (Duncan and Razzell, 1972; Splittstoesser et al., 1980; Zhao et al., 1997). Thus, thepresence of coliforms or faecal coliforms on raw fruits and vegetables does not necessarily providean index of faecal contamination. Microorganisms capable of causing human disease can,however, be found on raw produce (Table 1), and should be viewed as a threat to public health(Beuchat, 1996b; Doyle, 1990). What is not well known is where contamination occurs along thefarm-to-fork pathway. Means to address this issue must address the entire system from productionto consumption.

The presence of pathogenic and other microorganisms on fruits and vegetables is dictated notso much by surface topography as by exposure to environmental factors that lead to contamination.Although rough, highly-textured surfaces with deep crevices would be more likely to harbour soil,with the possible consequence of increased numbers of microorganisms, than would smooth-surfaced fruits and vegetables, differences in microbial profiles result largely from unrelated factorssuch as resident microflora in the soil, application of non-resident microflora via animal manures,sewage or irrigation water, rainfall and atmospheric humidity. The presence of a pathogen onproduce is of less consequence if the rind, skin or peel is to be removed before consumption.Bananas, melons, mangoes, durians, pineapples and papayas, for example, fall in this category.However, the process of removing the rind, skin or peel from these fruits, perhaps with theexception of bananas, may result in contamination of the edible portion, thereby creating a risk tothe consumer. Microorganisms that have become trapped on the inner leaves of certain vegetablescan be particularly difficult to remove by routine cleansing practices. Brief descriptions ofpathogens that have been isolated from raw fruits and vegetables are given below, with specialemphasis on their association with outbreaks of human disease.

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Table 2. Examples of pathogens associated with fruits and vegetables involved inoutbreaks of foodborne disease

Agent Implicated/ Referencesuspected food

Bacillus cereus sprouts Portnoy et al. (1976)Campylobacter cucumber Kirk et al. (1997)Campylobacter jejuni lettuce CDC (1998)Clostridium botulinum vegetable salad PHLS (1978)Cryptosporidium apple cider CDR (1991)Cyclospora raspberries Herwaldt et al.(1997)E.coli O157 radish sprouts WHO (1996)E.coli O157 apple juice CDC (1996)E.coli O157 apple cider Besser et al. (1993)E.coli O157 iceberg lettuce CDR(1997)Fasciolia hepatica watercress Hardman (1970)Giardia vegetables, incl.carrots Mints et al. (1992)Hepatitis A virus iceberg lettuce Rosenblum et al.(1990)Hepatitis A virus raspberries Ramsay et al. (1989)Hepatitis A virus strawberries Niu et al. (1992)Norwalk virus tossed salad Lieb et al. (1985)Salmonella Agona coleslaw & onions Clark et al. (1973)Salmonella Miami watermelon Gayler et al. (1955)Salmonella Oranienburg watermelon CDC (1979)Salmonella Poona cantaloupes CDC (1991)Salmonella Saint-Paul beansprouts O�Mahony et al. (1990)Salmonella Stanley alfalfa sprouts Mahon et al.(1997)Salmonella Thompson root vegetables & dried seaweed Kano et al.(1996)Shigella flexneri mixed salad Dunn et al. (1995)Shigella sonnei iceberg lettuce Kapperud et al. (1995)Shigella sonnei tossed salad Martin et al. (1986)Vibrio cholerae salad crops & vegetables Shuval, et al. (1989)

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Pathogens of most concern

Salmonella

The antigenic scheme for classifying salmonellae recognizes more than 2300 serovars and,while all can be considered human pathogens, only about 200 are associated with human illness.Animal husbandry practices used in the poultry, meat and fish industries, and the recycling of offaland inedible raw materials into animal feeds, has favoured the continued prominence ofSalmonella in the global food chain (D�Aoust, 1997). There are reports of human salmonellosislinked to cantaloupe (Ries et al., 1990) and sprouts produced from alfalfa seeds (Mahon et al., 1996)imported to the United States. Hygienic conditions during the production, harvesting, transport anddistribution of raw fruits and vegetables from some countries may not always meet minimumhygienic requirements, thus facilitating contamination on arrival in another country. Application ofnight soil, untreated sewage sludge or effluents, or irrigation water containing untreated sewage tofields and gardens can result in contamination of fruits and vegetables with Salmonella and otherpathogens. Washing fruits and vegetables with contaminated water and handling of produce byinfected workers, vendors and consumers in the marketplace helps the spread of pathogenicmicroorganisms, including Salmonella.

Salmonellae have been isolated from many types of raw fruits and vegetables (Beuchat, 1996b;Wells and Butterfield, 1997). Outbreaks of salmonellosis have been linked to a diversity of fruitsand vegetables, including tomatoes (Centers for Disease Control and Prevention, 1993; Hedberg etal., 1993; Wood et al., 1991), bean sprouts (Mahon et al., 1996; O�Mahony et al., 1990; VanVenedey et al., 1996), melons (Blostein, 1991; Centers for Disease Control and Prevention, 1979;1991; Gaylor et al., 1955; Ries et al., 1990), unpasteurized orange juice (Cook et al., 1990) andapple juice (Centers for Disease Control and Prevention, 1975). The pathogen can grow on thesurface of alfalfa sprouts (Jaquette et al., 1996), tomatoes (Zhuang et al., 1995) and perhaps on othermature raw fruits and vegetables, making it imperative to use hygienic practices when handlingthem.

Shigella

Bacillary dysentery or shigellosis is caused by Shigella, of which there are four species: S.dysenteriae, S. flexneri, S. boydii and S. sonnei (Maurelli and Lampel, 1997). Most cases ofshigellosis result from the ingestion of food or water contaminated with human faeces. Likesalmonellae and other pathogens present in faeces, Shigella can contaminate raw fruits andvegetables by several routes, including insects and the hands of persons who handle the produce,although shigellosis is more often transmitted from person to person.

Several large outbreaks of shigellosis have been attributed to the consumption of contaminatedraw fruits and vegetables. Lettuce (Davis et al., 1988; Frost et al., 1995; Kapperud et al., 1995;Martin et al., 1986), scallions (Cook et al., 1995), vegetable salad (Dunn et al., 1995), potato saladcontaining spring onions (Formal et al., 1965), salad vegetables (Public Health Research Service,1997) and watermelon (Frelund et al., 1987) have been implicated as vehicles of shigellosis. Sliced

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raw papaya, jicama and watermelon support the growth of Shigella species (Escartin et al., 1989).S. sonnei survived on refrigerated, shredded lettuce for three days without decreasing in number,and increased when held at 22°C (Satchell et al., 1990).

Escherichia coli

Escherichia coli is common in the normal microflora of the intestinal tracts of humans and otherwarm-blooded animals. Strains that cause diarrhoeal illness are categorized into groups on thebasis of virulence properties, mechanisms of pathogenicity, clinical syndromes and antigeniccharacteristics. The major groups are designated as enterotoxigenic, enterohaemorrhagic,enteropathogenic, enteroinvasive, diffuse-adhering and enteroaggregative (Doyle et al., 1997).Fruits and vegetables can become contaminated with one or more of these groups while in the fieldor during post-harvest handling. Sources and mechanisms of contamination are similar to thosedescribed for Salmonella and Shigella.

Enterotoxigenic E. coli is a cause of traveller�s diarrhoea, an illness sometimes experiencedwhen individuals visit countries with food and water hygiene standards different from their own.Contaminated raw vegetables are thought to be a common cause of traveller�s diarrhoea. Illness hasbeen associated with consumption of salads (Merson et al., 1976; Mintz, 1994) and carrots (Centersfor Disease Control and Prevention, 1994). Enterohaemorrhagic E. coli O157:H7 has more recentlybeen recognized as a foodborne pathogen. Since cattle appear to be a natural reservoir for thepathogen, most outbreaks of illness have been associated with the consumption of contaminated,undercooked beef and dairy products. However, outbreaks have also been linked to lettuce (Ackerset al., 1996; Mermin et al., 1996), apple cider (Besser et al., 1993; Centers for Disease Control andPrevention, 1996; Steele et al., 1982), radish sprouts (Nathan, 1997) and alfalfa sprouts (Centers forDisease Control and Prevention, 1997e). Enterohemorrhagic E. coli can grow on cantaloupe andwatermelon cubes (del Rosario and Beuchat, 1995), shredded lettuce (Diaz and Hotchkiss, 1996)and sliced cucumbers (Abdul-Raouf et al., 1993), and in apple cider (Zhao et al., 1993).

Contamination of raw fruits and vegetables with enterohaemorrhagic E. coli O157:H7 mayoccur when cattle, and perhaps other ruminants such as deer, inadvertently enter fields, or whenimproperly composted cow manure has been applied as a fertilizer. The potential forcontamination may be enhanced when fruits or vegetables have fallen from the plant to the groundand are then picked and placed into the handling and processing chain. Also, becausecontaminated manure may become airborne dust particles, it is possible that fruits on trees andvines may become contaminated. Workers on farms and in packing houses may also be a sourceof E. coli O157:H7. These mechanisms of contamination are somewhat speculative at present andmust be thoroughly investigated before appropriate interventions can be introduced to reduce therisk.

Campylobacter

Campylobacter jejuni is a leading cause of bacterial enteritis in many countries. Reservoirs ofthis pathogen include several wild animals as well as poultry, cows, pigs and domestic pets(Nachamkin, 1997). While consumption of food of animal origin, particularly poultry, is largelyresponsible for infection, Campylobacter enteritis has also been associated with the consumptionof raw fruits and vegetables (Bean and Griffin, 1990; Harris et al., 1996). Although Campylobacterdoes not grow at temperatures below 30 °C and is sensitive to acid pH, it can survive on cut fruitsfor sufficient time to be a risk to the consumer (Castillo and Escartin, 1994).

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Yersinia enterocolitica

Yersinia enterocolitica can be found in a variety of terrestrial and freshwater ecosystems,including soil, vegetation and water in lakes, rivers, wells and streams (Kapperud, 1991), but mostisolates from these sources lack virulence for humans. Pigs, however, frequently carry serotypescapable of causing human disease. The ability of Y. enterocolitica to grow at refrigerationtemperature and its documented presence on raw produce raises concern about the potential ofsalad vegetables as causative vehicles of yersiniosis in humans. Seven per cent of carrot samplesobtained from eating establishments in France were reported to contain serotypes of Yersinia thatmay be pathogenic to humans (Catteau et al., 1985). In another study (Darbas et al., 1985), 50% ofraw vegetables analysed contained nonpathogenic strains of Yersinia. Incidence was higher on rootand leafy vegetables than on tomatoes or cucumbers. Certainly, application of improperlycomposted pig manure to vegetable fields should be avoided to reduce the possibility of pathogenicstrains being present on produce when it reaches the consumer.

Listeria monocytogenes

Listeria monocytogenes is present in the intestinal tract of many animals, including humans, soit is not surprising that the organism can also be found in the faeces of these animals, on the landthey occupy, in sewage, in soils to which raw sewage is applied and on plants which grow in thesesoils (Van Renterghem et al., 1991). The organism also exists in nature as a saprophyte, growing ondecaying plant materials, so its presence on raw fruits and vegetables is not rare (Beuchat, 1992;1996a; Beuchat et al, 1990). Surveys of fresh produce have revealed its presence on cabbage,cucumbers, potatoes and radishes in the United States (Heisick et al., 1989), ready-to-eat salads inthe United Kingdom (Sizmur and Walker, 1988), the Netherlands (Beckers et al., 1989), NorthernIreland (Harvey and Gilmour, 1993) and Canada (Odumeru et al., 1997), tomatoes and cucumbersin Pakistan (Vahidy, 1992), and bean sprouts, sliced cucumbers and leafy vegetables in Malaysia(Arumugaswamy et al., 1994).

A cabbage-associated outbreak of listeriosis has been documented (Schlech et al., 1983).Listeria monocytogenes grows at temperatures as low as 2°C (Rocourt and Cossart, 1997) and canthrive in cool, wet areas in processing facilities. It can also grow on endive (Carlin, 1994; Carlin etal., 1995), lettuce (Beuchat and Brackett, 1990a), tomatoes (Beuchat and Brackett, 1991),asparagus, broccoli and cauliflower (Berrang et al., 1989b) and cabbage (Beuchat et al., 1986) butappears to be inhibited by carrot juice (Beuchat and Brackett, 1990b; Beuchat et al., 1994; Beuchatand Doyle, 1995; Nguyen-the and Lund, 1991; 1992). Production of 6-methoxymellein and 6-hydroxymellein by carrot cells infected by fungi or upon partial hydrolysis is known to occur (Aminet al., 1986; Kurusaki and Nishi, 1984). These phytoalexins inhibit a wide range of spoilage andpathogenic bacteria. The mechanism of action of 6-methoxymellein apparently involvesinterference with membrane-associated functions. Controlled atmosphere storage has been shownto extend the shelf-life of broccoli and asparagus but does not influence the rate of growth of L.monocytogenes (Berrang et al., 1989b). The risk of listeriosis increases when these vegetables arestored for longer periods before consumption because L. monocytogenes has a greater opportunityto increase.

Staphylococcus aureus

Staphylococcus aureus is known to be carried in the nasal passages of healthy food handlers andhas been detected on raw produce (Abdelnoor et al., 1983) and ready-to-eat vegetable salads

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(Houang et al., 1991). However, enterotoxigenic S. aureus does not compete well with othermicroorganisms normally present on raw fruits and vegetables, so spoilage caused bynonpathogenic microflora would probably precede the development of the high populations of thispathogen that would be needed for production of staphylococcal enterotoxin.

Clostridium species

Spores of Clostridium botulinum and Clostridium perfringens can be found both in soil and onraw fruits and vegetables. The high rate of respiration of salad vegetables can create an anaerobicenvironment in film-wrapped packages, thus favouring the growth of C. botulinum and botulinaltoxin production. Botulism has been linked to coleslaw prepared from packaged, shreddedcabbage (Solomon et al., 1990) and chopped garlic in oil (St. Louis et al., 1988). Studies haverevealed that C. botulinum can produce toxin in polyvinyl film-packaged (Sugiyama and Yang,1975) and vacuum-packaged mushrooms (Malizio and Johnson, 1991). It is important that thepermeability characteristics of packaging films minimize the possibility of development ofanaerobic conditions suitable for outgrowth of clostridial spores. Recognizing that anaerobicpockets may develop in tightly packed produce, even when films have high rates of oxygen andcarbon dioxide permeability, an additional measure to prevent growth of C. botulinum is to storeproduce at less than 3°C.

Bacillus cereus

Spores of enterotoxigenic strains of Bacillus cereus are common in most types of soil. Somestrains can grow at refrigeration temperatures. Foods other than raw fruits and vegetables aregenerally linked to illness implicating B. cereus. Illness associated with eating contaminated soy,mustard and cress sprouts has, however, been documented (Portnoy et al., 1976). Human illnesstends to be restricted to self-limiting diarrhoea (enterotoxin) or vomiting (emetic toxin). However,emetic toxin-producing strains have produced liver failure and death by the foodborne route.

Vibrio species

Vibrio species are generally the predominant bacterial species in estuarine waters and aretherefore associated with a great variety of fish and seafoods. There are 12 human pathogenicVibrio species, of which Vibrio cholerae, V. parahaemolyticus and V. vulnificus are of greatestconcern (Oliver and Kaper, 1997). Vibrio cholerae is the causative agent of cholera, one of the fewfoodborne diseases with epidemic and pandemic potential. Carriage of the organism by infectedhumans is important in transmission of disease. Water can become contaminated by raw sewage.Ingestion of water containing V. cholerae or of foods that are washed with contaminated water butnot disinfected can lead to widespread transmission of cholera (Mintz et al., 1994). An outbreak ofcholera linked to the consumption of unpasteurized coconut milk has been documented (Taylor etal., 1993).

Vibrio parahaemolyticus is perhaps the best described of the pathogenic vibrios in terms of itsinvolvement in foodborne illness. Outbreaks are generally associated with consumption ofcontaminated raw or undercooked seafoods and are seasonal, peaking in summer when V.parahaemolyticus is at its highest population in estuarine water. Cross-contamination of raw fruits

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and vegetables with seafoods during handling, particular at retail locations, represents a potentialmode of transmission to humans.

Of all the vibrios, V. vulnificus infection from contaminated seafoods results in the highestfatality rate, typically 60% in individuals suffering from liver diseases (Oliver and Kaper, 1997). Aswith V. cholerae and V. parahaemolyticus, it is possible that cross-contamination of raw fruits andvegetables with V. vulnificus could occur, resulting in human infection when they are consumed.

Viruses

Viruses can be excreted in large numbers by infected individuals (Cliver, 1997). Althoughviruses will not grow in or on foods, raw fruits and vegetables may serve as vehicles for infection.Many food-associated outbreaks of hepatitis A have been recorded (Cliver, 1997). In mostinstances, these outbreaks have not appeared to depend on the stability of the virus in the food.Shellfish taken from waters contaminated with human faeces have been the vehicle in mostoutbreaks, but any food handled by an infected person may become contaminated and transmitinfection (Cliver, 1985). Hepatitis A infection has been linked to the consumption of lettuce(Rosenblum et al., 1990), diced tomatoes (Williams et al., 1994), raspberries (Ramsay and Upton,1989; Reid and Robinson, 1987) and strawberries (Centers for Disease Control and Prevention,1997a; Niu et al., 1992). Hernandez et al. (1997) suggested that lettuce contaminated with sewagecould be a vehicle for hepatitis A virus and rotavirus. Lettuce obtained from farmers� markets wasreported to contain hepatitis A virus. The extent to which hepatitis A and other viruses are removedfrom the surface of fruits and vegetables upon treatment with chemical disinfectants is not known.

The number of cases of foodborne disease caused by Norwalk-like viruses (i.e. Small RoundStructured Viruses, or SRSV) appears to be on the increase (Bean and Griffin, 1990). Outbreaks havea pattern of transmission resembling that of hepatitis A. Ice made from contaminated water has beenimplicated as the vehicle in more than one outbreak but salad items have also been linked toNorwalk-like gastroenteritis (Karitsky et al., 1995). Workers who have prepared salads linked toviral gastroenteritis have been shown to have high antibody titers to Norwalk virus (Griffin et al.,1982; Gross et al., 1989; Iverson et al 1987). A non-typical outbreak of Norwalk virus gastroenteritisassociated with exposure of celery to nonpotable water has been reported (Warner, 1991). Studieshave shown that viruses may persist for weeks or even months on vegetable crops and in soils thathave been irrigated or fertilized with sewage wastes (Larkin et al., 1978).

Rotaviruses, astroviruses, enteroviruses (polioviruses, echoviruses and coxsackie viruses),parvoviruses, adenoviruses and coronaviruses have been reported to be transmitted by foods onoccasion (Cliver, 1994). At least one echovirus outbreak has been attributed to contaminated rawshredded cabbage (New York Department of Health, 1989).

Parasites

The major modes of transmission of protozoa include consumption of surface water, exposureto contaminated recreational water, animal-to-person contact and person-to-person contact(Speer, 1997). However, the epidemiology of protozoa most commonly associated with humaninfections, namely Giardia, Entamoeba, Toxoplasma, Sarcocystis, Isopora, Cryptosporidium,Eimeria and Cyclospora, is not fully understood (Goodgame, 1996). While the life cycles of eachof these parasites differ, all require passage through an animal or human host. Shedding of cysts or

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spores into faeces which may then, directly or indirectly (e.g. via sewage or irrigation water),contaminate raw fruits and vegetables occurs on a global scale; it may be common in countrieswhere hygienic conditions (especially water quality) are compromised. Expanding poverty, urbanmigration, food exportation, air travel and immigrant populations in conjunction with other societaland environmental changes make the distinction between so-called tropical and western parasiticdiseases less clear-cut than in the past.

Outbreaks of protozoan infections in humans have been linked to raw fruits and vegetables.Epidemiological evidence has implicated an asymptomatic food handler as the probable source ofGiardia lamblia and raw sliced vegetables as the vehicle of transmission in an outbreak of giardiasis(Mintz et al, 1993). Raspberries (Centers for Disease Control and Prevention, 1996c; 1997b,c;Jackson et al., 1997), lettuce (Centers for Disease Control and Prevention, 1997c), and basil(Centers for Disease Control and Prevention, 1997d) have been the implicated vehicles oftransmission in outbreaks of Cyclospora cayetanensis infection, and unpasteurized apple juice hasbeen linked to outbreaks of cryptosporidiosis caused by Cryptosporidium parvum (Millard et al.,1994). A survey of vegetables has revealed the presence of Cryptosporidium oocysts on cilantro,lettuce, radish, tomato, cucumber and carrot (Monge and Chinchilla, 1996). The current status ofknowledge about Cryptosporidium and its significance in food and beverage production has beenreviewed by Donnelly and Stentiford (1997). The presence of these protozoa on raw fruits andvegetables is likely to be due to contact with animal or human faeces, sewage, water containinguntreated sewage and sludge from primary or secondary municipal water treatment facilities.While Cryptosporidium, Giardia and other parasites in water are quite resistant to chlorine andother disinfectants, little is known about the efficacy of these disinfectants in killing or removingparasites from the surface or tissues of fruits and vegetables. Surveys have shown that there is a highincidence of the parasitic roundworm, Ascraris, in the sewage sludge of many cities (Jackson et al.,1977).

Foodborne trematode infections are major health problems, with an estimated 40 millionpersons, mainly in eastern and southern Asia, being affected (Abdussalam et al., 1995). Infectiontakes place through the consumption of raw plants, or raw or undercooked freshwater fish orshellfish, containing the infective cyst (metacercaria) stage of these parasites. Watercress is a majorsource of Fasciola hepatica infection (World Health Organization, 1995). Over 300,000 clinicalcases of fascioliasis may have occurred in more than 55 countries in Africa, the Americas, Asia,Europe and the western Pacific from 1970 to 1990 (Chen and Mott, 1990). Large endemic areashave been reported more recently in Bolivia, Egypt, Iran and Peru (Abdussalam et al., 1995).

A wide range of other aquatic plants may support metacercariae. Conditions for transmission ofFasciolopsis buski are present in areas of cultivation of water caltrop, water chestnut, waterhyacinth, water bamboo, water mimosa, lotus and duckweed. If animal manure or effluent fromlivestock pens or abattoirs is used as fertilizer for these plants, it introduces Fasciolopsis to theaquatic environment. Plantborne trematodes encyst as metacerceriae on the surface of plants or ondebris floating on the water surface. Plants that grow in water are also believed to serve as hosts forencystment of metacercariae of certain intestinal flukes (World Health Organization, 1995). Non-aquatic plants such as lettuce, alfalfa, mint and sugarcane which may be eaten raw have also beenimplicated in human trematode infections.

Strategies for controlling foodborne trematode infections associated with eating raw plantmaterials will be shaped by ecological and environmental factors. The use of non-compostedanimal manures or effluent from livestock pens for the purpose of fertilizing aquatic plants shouldcease. Freezing, heating and irradiation can be effective in killing trematodes. The effectiveness ofchemical treatments in killing trematodes attached to plant surfaces is not well understood.

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Mitigating the risk of disease

To minimize the risk of infection or intoxication associated with raw fruits and vegetables,potential sources of contamination from the environment to the table should be identified andspecific measures and interventions to prevent and/or minimize the risk of contamination

should be considered and correctly implemented. Where the possibility of contamination cannotbe excluded, the application of the most effective decontamination processes should beconsidered. In many regions of the world, present practices in agriculture practice cannot assurefruits and vegetables that are free from pathogens, as the studies mentioned in Table 1 illustrate.Application of good hygienic practice during production, transport and processing, combined withthe Hazard Analysis Critical Control Point system, will certainly minimize the contamination offruits and vegetables and reduce the risk of illness associated with these foods. However, foodhandlers and consumers also need to observe good hygienic practice during processing andpreparation of these foods for consumption � including treatments for reducing the number ofpathogens.

The simple practice of washing raw fruits and vegetables in hot water or water containingdetergent or permanganate salts removes a portion of the pathogenic and spoilage microorganismsthat may be present, but studies showing the efficacy of these treatments are few. Even washingfruits and vegetables in potable water, then again washing or rinsing in potable water would aidein removing microorganisms. Additional 10-fold to 100-fold reductions can sometimes beachieved by treatment with disinfectants. Viruses and protozoan cysts on fruits and vegetablesgenerally exhibit higher resistance to disinfectants than do bacteria or fungi. However, relativeresistance varies greatly with the type and pH of disinfectant, contact time, temperature and thechemical and physical properties of the fruit or vegetable surface. Little is known about the efficacyof disinfectants in relation to the roughness of fruit and vegetable surfaces, although higher amountsof cuticle material may protect against embedding of cells, thereby increasing the need for exposureto chemical treatments.

Several types of treatment are known to be partially effective in removing disease-causingorganisms from the surface of whole and cut raw fruits and vegetables or from contact surfacesduring handling. Perhaps with the exception of irradiation, none of these treatments can be reliedupon to totally disinfect raw produce, at least when administered at levels that will not causedeterioration in sensory quality. Even irradiation may not be completely effective in killing viruseson fruits and vegetables. Rather, these treatments should be considered as methods of disinfection,causing reductions in populations of microorganisms but not always yielding fruits and vegetablesfree of pathogens.

Each type of dis infectant has its own efficacy in killing microbial cells. Effectiveness depends onthe nature of the cells as well as the characteristics of fruit and vegetable tissues and juices. Sometypes of disinfectants are appropriate for use in direct contact washes, while others are suitable onlyfor equipment or containers used to process, store or transport fruits and vegetables. Themechanism of action of many disinfectants on microbial cells and the influence of factorsassociated with plant materials is poorly understood.

The legal use of various treatments differs from country to country. Described here aredisinfectants and disinfection treatments that are in current use or that have potential for killingmicroorganisms on whole and cut raw fruit and vegetables, as well as surfaces that are in contactwith produce during handling.

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Chlorine

Chlorine has been used for many years to treat drinking-water and wastewater as well as tosanitize food processing equipment and surfaces in processing environments. It is also used as adisinfectant in wash, spray and flume waters in the raw fruit and vegetable industry. Chlorinationis generally accomplished by using elemental chlorine or one of the hypochlorites. Liquid chlorineand hypochlorites are moderately effective disinfectants for surfaces that may come in contact withfruits and vegetables during harvesting and handling, for processing equipment, and for whole andcut fruits and vegetables. To disinfect produce, chlorine is commonly used at concentrations of 50-200 ppm with a contact time of 1-2 minutes.

Inhibitory or lethal activity depends on the amount of free available chlorine (as hypochlorousacid, or HOCl) in the water that comes in contact with microbial cells. The dissociation of HOCldepends on the pH, and chlorine is consumed on contact with organic matter. As the pH of thesolution is reduced, the equilibrium is in favour of HOCl. However, since metal containers andprocessing equipment are often susceptible to corrosion at low pH, a pH of 6.0-7.5 is mostappropriate for effective sanitizing activity without damaging equipment surfaces. The percentagesof chlorine as HOCl at pH 6.0 and 8.0 are about 97% and 23%, respectively, at 20°C. Toxicchlorine gas (Cl2) is formed at a pH below 4. At a given pH, equilibrium is in favour of HOCl as thetemperature is decreased. This is because chlorine vaporizes as water temperature increases.Chlorine rapidly loses activity on contact with organic matter or exposure to air, light or metals. Aconcern among people who use chlorinated water as a disinfectant is that prolonged exposure tochlorine vapours can cause irritation to the skin and respiratory tract. The occupational exposurelimit (ceiling) is 1 ppm in the United States (instantaneous up to 15 minutes) (OSHA). The formationof potentially hazardous organochlorines upon treatment of fruits and vegetables with chlorine isalso a possible concern, although these have not been characterized.

Maximum solubility of chlorine is achieved in water at about 4°C. However, the temperature ofthe chlorinated water should ideally be at least 10°C higher than that of fruits or vegetables toachieve a positive temperature differential, thereby minimizing the uptake of wash-water throughstem tissues (Bartz and Showalter, 1981; Zhuang et al., 1995) and open areas in the skin or leaves,whether due to mechanical assault or naturally present (e.g. lenticels and stomata). Elimination ofuptake of wash-water that may contain microorganisms, including those that may cause humanillnesses, should be considered as a critical control point in handling, processing and disinfectionof raw fruits and vegetables.

Possible uses of chlorinated water in packing-houses and during washing, cooling and transportfor the purpose of controlling post-harvest diseases of fruits and vegetables have been reviewed byEckert and Ogawa (1988). The effects of chlorine concentration on aerobic microorganisms andfaecal coliforms present on leafy salad greens was studied by Mazollier (1988). Populations ofpathogens were markedly reduced with increased concentrations of chlorine to 50 ppm, but furtherincreases in concentration to 200 ppm did not have a substantial additional effect. A standardprocedure for washing lettuce leaves in tap-water reduced populations (ca. 10 7/g) of microflora by92% (Adams et al., 1989). Inclusion of 100 ppm chlorine (pH 9.0) reduced the count by 97.8%,indicating that this concentration of chlorine in treatment water was only slightly more effectivethan using water with no chlorine. Adjusting the pH from 9 to 4.5-5.0 with inorganic and organicacids resulted in a 1.5-4.0-fold increase in microbicidal effect. Increasing the washing time inhypochlorite solution from 5 to 30 minutes did not decrease numbers of microbes further, whereasextended washing in tap-water resulted in a reduction comparable to hypochlorite. The addition of100 ppm of a surfactant (Tween 80) to hypochlorite washing solution enhanced lethality byenhancing surface contact but adversely affected sensory qualities of lettuce. Somers (1963)

17


reported that wash-water with about 5 ppm chlorine reduced microbial populations on severalfruits and vegetables by less than 90% from initial populations of 104-106 CFU/g.

In addition to the influence of pH and temperature on the effectiveness of chlorine in killingmicroorganisms that occur naturally on fruits and vegetables, the type of produce and diversity ofmicroorganisms can also greatly influence efficacy. Garg et al. (1990), for example, observed thatdipping lettuce in water containing 300 ppm chlorine reduced total microbial counts by about1000-fold on lettuce but had no effect on microbial counts on carrots or red cabbage. Treatment ofwhole and shredded lettuce leaves in water containing 200-250 ppm chlorine reduced populationsof aerobic microorganisms by 90-99% and psychotrophic microorganisms and yeasts and mouldsby 50-90%.

Studies have aimed to determine the effectiveness of chlorine in killing bacterial pathogensinoculated onto the surface of raw vegetables. Dipping Brussels sprouts into a 200-ppm chlorinesolution for 10 seconds decreased the number of viable Listeria monocytogenes cells (106 CFU/g)by about 100-fold (Brackett, 1987). However, dipping inoculated sprouts in sterile watercontaining no chlorine reduced the number of viable cells by about 10-fold. The maximum log10

reduction of L. monocytogenes on shredded lettuce and cabbage treated with 200 ppm chlorine for10 minutes was reported to be 1.3-1.7 log10 CFU/g and 0.9-1.2 log10 CFU/g respectively (Zhang andFarber, 1996). Initial populations ranged from log10 5.4 to 5.7 CFU/g. Reductions were greater whentreatment was at 22°C than at 4°C, perhaps because the temperature of lettuce and cabbage wasless than 22°C but more than 4°C. As noted above, the effectiveness of chlorine in killingmicroorganisms is generally greater if the temperature of the treatment solution is higher than thetemperature of the fruit or vegetable. Treatment was more effective at both temperatures against L.monocytogenes on lettuce than on cabbage, indicating that antimicrobial activity is influenced bythe nature of the vegetable being treated and/or the microflora it harbours. Numbers decreased onlymarginally with increased exposure time from 1 to 10 minutes, which agrees with observations byBrackett (1987) that the action of chlorine against L. monocytogenes occurs primarily during thefirst 30 seconds of exposure. Nguyen-the and Carlin (1994) concluded that the reduction of L.monocytogenes from the surface of vegetables by chlorine is both unpredictable and limited.

The efficacy of chlorine treatment on inactivation of Salmonella Montevideo on mature greentomatoes has been studied. Populations on the surface and in the stem core tissue were significantlyreduced by dipping tomatoes for 2 minutes in a solution containing 60 ppm or 110 ppm chlorinerespectively. However, treatment in a solution containing 320 ppm chlorine did not result incomplete inactivation (Zhuang et al., 1995). The ineffectiveness of 100 ppm chlorine against S.Montevideo inoculated into cracks in the skin of mature green tomatoes was demonstrated by Weiet al. (1995).

Immersion of warm (26-40°C) tomatoes for 10 minutes or longer in cool (20-22°C) suspensionsof bacteria resulted in infiltration of cells into the stem tissue (Bartz and Showalter, 1981). Uptakeof bacterial cells was associated with a negative temperature difference between the water and thetomato, i.e. the water temperature was less than the tomato temperature. When the differential wasshifted to a positive relationship, i.e. when the water temperature was higher than the tomatotemperature, the extent of infiltration was reduced. A significantly higher number of S. Montevideocells has been shown to be taken up by the core tissue when tomatoes at 25°C are dipped insuspensions at 10°C compared with the number of cells taken up by tomatoes dipped insuspensions at 25 or 37°C (Zhuang et al., 1995). Thus, the concentration of free chlorine reachingviable cells of S. Montevideo that have infiltrated the core tissues is reduced to the point thatlethality is substantially diminished. The uptake of pathogens and spoilage microorganisms byother fruits and vegetables during washing has not been investigated. However, infiltration of

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microbial cells due to a negative temperature differential between the water and the fruit orvegetable would appear to be possible.

The effectiveness of chlorine in killing salmonellae on alfalfa seeds has been studied. Treatmentof seeds inoculated with Salmonella Stanley (102-103 CFU/g) in 100 ppm chlorine solution for 10minutes has been reported to cause a significant reduction of the pathogen; treatment in 290 ppmchlorine solution resulted in a significant reduction compared to treatment with 100 ppm chlorine(Jaquette et al., 1996). However, solutions containing free chlorine concentrations up to 1000 ppmfailed to result in further significant reductions. Treatment of seeds containing 101-102 CFU of S.Stanley per g for 5 minutes in a solution containing 2040 ppm chlorine reduced the population to<1 CFU/g. The sensory quality of sprouts produced from seeds receiving this treatment is notadversely affected.

In another study, alfalfa sprouts inoculated with a five-serovar (S. Agona, S. Enteritidis, S.Hartford, S. Poona, S. Montevideo) mixture of Salmonella were dipped in 200, 500 or 2000 ppmchlorine solutions for 2 minutes (Beuchat, 1997). The pathogen was reduced by about 3.4 log10

CFU/g after treatment with 500 ppm chlorine and to an undetectable level (<1 CFU/g) aftertreatment with 2000 ppm chlorine. Chlorine treatment (2000 ppm) of cantaloupe cubes inoculatedwith the same Salmonella serovars resulted in less than a 90% reduction in viable cells. The veryhigh level of organic matter in the juice released from cut cantaloupe tissue apparently neutralizesthe chlorine before its lethality can be manifested.

Failure to maintain adequate chlorine in wash-water may lead to increases in microbialpopulations on fruits and vegetables. In a study designed to determine microbiological changes infresh market tomatoes during packing operations, Senter et al. (1985) observed that total platecounts and populations of Enterobacteriaceae were higher, compared to controls, on tomatoeswashed in water containing an average of 114 ppm (range 90-140) chlorine; decreases were notedwhen tomatoes were treated in water containing 226 ppm chlorine (range 120-280).Recontamination of tomatoes occurred in the waxing operation as evidenced by increased totalplate counts and mould populations.

Fruit and vegetable tissue components neutralize chlorine, rendering it inactive againstmicroorganisms. The inaccessibility of hypochlorous acid to microbial cells in cracks, creases,crevices, pockets and natural openings in the skin undoubtedly also contributes to chlorine�soverall lack of effectiveness. The hydrophobic nature of the waxy cuticle on the surface of fruits andvegetables protects microbial cells from exposure to chlorine and, undoubtedly, other chemicalsused as disinfectants that do not penetrate or dissolve these waxes. Surface-active agents such asdetergents and ethanol lessen the hydrophobicity of fruit and vegetable skins as well as the surfacesof edible leaves, stems and flowers, but also tend to cause deterioration of sensory qualities (Adamset al., 1989; Zhang and Farber, 1996). Disinfectants that contain a solvent that would remove thewaxy cuticle layer, and with it surface contaminants, without adversely affecting sensorycharacteristics would hold greater potential in reducing microbial populations on the surface of rawfruits and vegetables. Such disinfectants may be limited to use on fruits and vegetables that are tobe further processed into juice or cut products, or on whole fruits, vegetables or plant parts that aredestined for immediate consumption, since removal of cuticle material will also hastendeterioration of sensory quality. Application of such disinfectants at point of use diminishes theimportance of this drawback.

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Chlorine dioxide

Chlorine dioxide (ClO2) has received attention as a disinfectant for fruits and vegetables, largelybecause its efficacy is less affected by pH and organic matter and it does not react with ammoniato form chloramines, as do liquid chlorine and hypochlorites. One disadvantage of ClO2 is that itis unstable, it must be generated on site and can be explosive when concentrated. Chlorine dioxidedecomposes at temperatures greater than 30°C when exposed to light. Compared to chlorine, fewerorganohalogens are formed as reaction products of ClO2. Its potential for use as a disinfectant hasbecome more attractive in recent years due to the development of technologies that permitshipment to areas of use instead of requiring on-site generation. The oxidizing power of ClO2 isreported to be about 2.5 times that of chlorine (Benarde et al., 1967) and its activity is not affectedby pH. Its mechanism of action involves disruption of cell protein synthesis and membranepermeability control.

In the United States, a maximum of 200 ppm ClO2 is permitted for sanitizing equipment for fruitand vegetable processing. Chlorine dioxide is authorized for use in washing whole fresh fruits andvegetables and shelled beans and peas with intact cuticles at a concentration not exceeding 5 ppm.For peeled potatoes, the maximum permitted wash concentration is 1 ppm. The use of ClO2 todisinfect fresh-cut fruits and vegetables is not currently permitted in the United States. Theoccupational exposure limit (ceiling) is 0.1 ppm in air in the United States (instantaneous up to 15minutes) (OSHA).

Compared to the information that is available on the effectiveness of chlorine as a disinfectantfor fruits and vegetables, much less is known about the efficacy of ClO2. Control of post-harvestfungal pathogens on pears (Spotts and Peters, 1980) and protozoa in water (Chen et al., 1985) hasbeen studied. In vitro tests with conidia and sporangiospores of several fungal pathogens of applesand other fruits demonstrated >99% mortality resulting from a 1-minute treatment in watercontaining 3 or 5 ppm ClO2 (Roberts and Reymond, 1994). Longer exposure times were necessaryto achieve similar mortalities by treatment with 1 ppm. Of the moulds tested, Botrytis cinerea andPenicillium expansum were least sensitive to ClO2. Treatment of belts and pads in a commercialapple and pear packing-house with 14-18 ppm ClO2 in a foam formulation resulted in significantlylower numbers of fungi. It was concluded that ClO2 has desirable properties as a sanitizing agentfor post-harvest decay management when residues of post-harvest fungicides are not desired or notallowed.

The efficacy of ClO2 in preventing build-up of microorganisms in water for handling cucumbersand on the microorganisms present in fresh cucumbers has been studied (Costilow et al., 1984). At2.5 ppm, ClO2 was effective in killing microorganisms in wash-water but, at concentrations up to105 ppm, failed to reduce the population of microorganisms present in or on fresh cucumbers. Itwas concluded that many microorganisms were so intimately associated with the cucumber fruitthat they were unaffected by chlorine and ClO2. Reina et al. (1995) evaluated the efficacy of ClO2

in controlling microorganisms in recycled water in a spray-type hydrocooler used to treat picklingcucumbers. Residual ClO2 at 1.3 ppm was found to optimally control (2-6 log10 CFU/ml reduction)the number of microorganisms in the water. At 0.95 ppm ClO2, the population was static, while at2.8 and 5.1 ppm the odour became excessive. Populations of microorganisms on and in cucumberswere not greatly influenced by ClO2, even at 5.1 ppm. It was concluded that the use of ClO2 in waterused to cool cucumbers seems to be an effective means of controlling microbial build-up, but thatit has little effect on the viability of microorganisms on cucumbers.

The effectiveness of ClO2 in killing L. monocytogenes inoculated onto the surface of shreddedlettuce and cabbage leaves has been studied (Zhang and Farber, 1996). A 10-minute exposure of

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lettuce to 5 ppm ClO2 caused a maximum reduction of 1.1 and 0.8 logs in numbers of L.monocytogenes at 4°C and 22°C, respectively, compared with a tap-water control. Similar resultswere obtained with cabbage. Thus, the maximum reduction in populations of L. monocytogenes onshredded lettuce and cabbage treated with ClO2 at target concentrations up to 5 ppm was onlyslightly more than 90%, which conforms with observations on the lack of effectiveness of ClO2 inkilling microorganisms on and in cucumbers (Costilow et al., 1984; Reina et al., 1995). As observedwith chlorine and other disinfectants, microorganisms differ greatly in their sensitivity to ClO2, andenvironmental conditions under which ClO2 is applied can greatly influence efficacy. Theeffectiveness of ClO2 in killing specific pathogenic microorganisms on specific types of fruits andvegetables deserves further research attention.

Bromine

Bromine has had limited use either alone or in combination with chlorine compounds in watertreatment programmes, but little is known about its effectiveness as a disinfectant for fruits andvegetables. Pseudomonas aeruginosa, a spoilage bacterium, appears to be more resistant tobromine than some other bacteria. Free bromine (200 ppm) does not kill P. aeruginosa within 15minutes at 24°C, but does kill E. coli, Salmonella Typhosa and Staphylococcus aureus (Gershenfeldand Witlin, 1949). Chlorine is more lethal than dibromodimethyl hydrantoin against B. cereusspores (Cousins and Allan, 1967) but is equally effective against Streptococcus faecalis (Ortenzioand Stuart, 1964). Others (Kristofferson, 1958; Shere et al., 1962) have observed that the additionof bromine to solutions containing chlorine compounds increases antimicrobial activity, theeffectiveness sometimes being synergistic. The occupational permissible exposure limit (PEL) in airin the United States is 0.1 ppm (8-hour time-weighted average) (OSHA).

Iodine

Like chlorine, iodine compounds are widely used for sanitizing food processing equipment andsurfaces. Under most conditions, free elemental iodine and hypoiodous acid are believed to be theactive antimicrobial agents (Odlaug, 1981). The major iodine formulations are ethanol-iodinesolutions, aqueous iodine solutions and iodophors, which are combinations of elemental iodineand nonionic surfactants such as nonyl-phenol ethoxylates (Bartlett and Schmidt, 1957) or a carriersuch as polyvinylpyrrolidone (Lacey, 1979). Iodophors have greater solubility in water, are lessvolatile and irritating to the skin than ethanolic or aqueous solutions of iodine (Lawrence et al.,1957), and have a broad spectrum of activity, including yeasts and moulds. Iodophors are lesscorrosive than chlorine at low temperatures but vapourize at temperatures above about 50°Cwhere they can be highly corrosive. The efficacy of iodophors is reduced at low temperatures.

Iodophors are most effective at pH 2-5 but can also be active at slightly alkaline pH, dependingon other conditions. At concentrations of 6-13 ppm available iodine (pH 6.6-7.0), the time toreduce populations of vegetative bacteria by 90% ranges from 3 to 15 seconds (Gray and Hsu,1979; Hays et al., 1967; Mosley et al., 1976). Bacterial spores are very resistant to iodine, comparedto vegetative cells. Spores of B. cereus, B. subtilis and Clostridium botulinum (type A) have D values(90% reduction in population) 10-fold to 1000-fold greater than vegetative cells of bacteria treatedwith 10-100 ppm iodophor (Odlaug, 1981). Although iodophors are minimally affected by organicmatter, they may stain equipment used to handle fruits and vegetables and react with starch to forma blue-purple colour. For this reason, the use of iodophors for direct-contact disinfection of manyfruits and vegetables has limited potential.

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Trisodium phosphate

Trisodium phosphate (TSP) is known to be effective in removing Salmonella from poultry(Lillard, 1994) and red meats (Dickson et al., 1994) when applied to carcasses in the form of a chillor rinse-water during processing. Zhuang and Beuchat (1996) investigated the effectiveness of TSPin wash-water in killing S. Montevideo on the surface and in core tissue of inoculated mature greentomatoes. Complete inactivation of Salmonella (5.18 log10 CFU/cm2) on the tomato surface wasachieved by dipping tomatoes in 15% TSP solution for 15 seconds. Significant (P 0.05) reductionswere obtained by dipping tomatoes in a 1% TSP solution for 15 seconds. Populations (5.58 log10

CFU/g) were significantly reduced in core tissue of tomatoes dipped in 4-15% TSP. However, evenat 15%, only about a 2 log10 reduction was achieved. It was concluded that the use of TSP as adisinfectant for removal of Salmonella from the surface of mature green tomatoes has goodpotential.

The use of TSP to remove L. monocytogenes from shredded lettuce is less promising. Zhang andFarber (1996) reported that treatment of lettuce with 2% TSP had almost no effect on reducing thepopulation of L. monocytogenes. Solutions containing more than 10% TSP damaged the sensoryquality of lettuce. Other investigators have observed that L. monocytogenes is resistant to TSP(Somers et al., 1994). Escherichia coli O157:H7, on the contrary, was sensitive to 1% TSP, 106 CFU/ml or 105 CFU/cm2 of biofilm being killed within 30 seconds at room temperature or 10°C.Campylobacter jejuni was only slightly more resistant than E. coli O157:H7. It should be noted thatthe pH of TSP solutions is in the 11 to 12 range, thus perhaps limiting their application as adisinfectant of fruits and vegetables to commercial use.

Quaternary ammonium compounds

Quaternary ammonium compounds (quats) are cationic surfactants used largely to sanitizefloors, walls, drains, and equipment and other food-contact surfaces in fruit and vegetableprocessing plants. Quats are noncorrosive to metals and stable at high temperature. They are moreeffective than chlorine against yeasts, moulds and gram-positive microorganisms such as L.monocytogenes, and less effective in killing gram-negative bacteria such as coliforms, Salmonella,pathogenic E. coli, Pseudomonas and Erwinia species, the latter two being among the majorspoilage bacteria of raw vegetables. However, the antimicrobial activity of quats variesconsiderably, depending on the type used. Because of their surfactant activity, quats have goodpenetrating ability and appear to form a residual antimicrobial film when applied to most hardsurfaces. Quats are relatively stable in the presence of organic matter and properly diluted solutionsare odourless and colourless. Their effectiveness is greatest in a pH range of 6-10, thus limiting theirpotential as disinfectants in highly acidic environments. Quats are not compatible with soaps andanionic detergents. Since most cleaners are anionic, surfaces must be throughly rinsed betweencleaning and disinfecting. This group of disinfectants has potential for application to surfaces ofuncut fruits and vegetables which subsequently would have their peel, rind or skin removed beforeconsumption.

Acids

Organic acids naturally present in fruits and vegetables or accumulated as a result offermentation are relied upon to retard the growth of some microorganisms and prevent the growthof others. Foodborne bacteria capable of causing human illness cannot grow at pH values less than

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about 4.0, so the acidic pH of the edible portions of most fruits precludes them as substrates forproliferation of human pathogens. The pH of many vegetables and a few fruits (e.g. melons),however, is the range at which pathogens can grow.

Some organic acids naturally found in or applied to fruits and vegetables behave primarily asfungistats, while others are more effective at inhibiting bacterial growth. Acetic, citric, succinic,malic, tartaric, benzoic and sorbic acids are the major organic acids that occur naturally in manyfruits and vegetables. The mode of action of these acids is attributed to direct pH reduction,depression of the internal pH of microbial cells by ionization of the undissociated acid molecule,or disruption of substrate transport by alteration of cell membrane permeability.

Washes and sprays containing organic acids, particularly lactic acid, have been successfullyused to disinfect beef, lamb, pork and poultry carcasses. Application of organic acid washes to thesurface of fruits and vegetables for the purpose of reducing populations of microorganisms also haspotential. Pathogenic microorganisms on the surface of uncut fruits and vegetables after washingwith water or a disinfectant solution, or transferred to the flesh or pulp of fruits or vegetables duringthe cutting process, can be killed or prevented from growing by applying organic acids. Proceduresas simple as applying lemon juice, which contains citric acid as the major acid, to cut fruits havebeen shown to kill or retard the growth of pathogens. Escartin et al. (1989) reported that applicationof lemon juice to the surface of papaya (pH 5.69) and jicama (pH 5.97) cubes inoculated withSalmonella Typhi reduced populations compared to the control, but growth resumed after severalhours. Survival of C. jejuni inoculated onto watermelon (pH 5.5) and papaya (pH 5.6) cubes asaffected by treatment with lemon juice was investigated by Castillo and Escartin (1994). Theamount surviving 6 hours after treatment ranged from 7.7% to 61.8% in fruits not treated withlemon juice acid and from 0% to 14.3% in fruits with lemon juice added. Treatment appeared tobe more effective in killing Campylobacter on papaya than on watermelon, perhaps because ofdifferences in porosity or buffer capacity of these fruits.

Shapiro and Holder (1960) observed that treatment of salad vegetables with up to 1500 ppmcitric acid did not affect bacterial growth during a subsequent 4-day storage period at 10°C.Treatment with 1500 ppm tartaric acid reduced total counts 10-fold. Treatment of cut lettuce,endive, carrots, celery, radishes and green onions with 2000 ppm sorbate or 10,000 ppm ascorbicacid, alone and in combination, resulted in less than 1 log difference in populations of aerobicmicroorganisms after 10 days of storage in 4.4°C (Priepke et al., 1976).

Removal of pathogenic bacteria from leafy salad greens by treating with acetic acid has beenstudied. Reduction in counts of Yersinia enterocolitica inoculated onto parsley leaves from 107

CFU/g to <1 CFU/g by washing in a solution containing 2% acetic acid or 40% vinegar for 15minutes was achieved by Karapinar and Gonul (1992). No viable aerobic bacteria were recoveredafter a 30-minute dip in 5% acetic acid, whereas a vinegar dip resulted in a 3-6 log10 decrease inthe number of aerobic bacteria, depending on vinegar concentration and holding time.

The effects of lactic and acetic acid, either alone or in combination with chlorine, on survival ofL. monocytogenes inoculated onto shredded lettuce were studied by Zhang and Farber (1996).Compared to lettuce washed in tap-water, only 1% lactic acid and combinations of 0.5% or 1%lactic acid and 100 ppm chlorine reduced numbers of L. monocytogenes. Lactic acid (0.75% or1%) in combination with 100 ppm chlorine was more effective in reducing levels of L.monocytogenes than were either lactic acid or chlorine alone. Results obtained with acetic acidwere similar to those with lactic acid.

Treatment of ready-to-use salads with 90 ppm peracetic acid has been shown to reduce totalcounts and faecal coliforms by nearly 100-fold, similar to reductions with 100 ppm chlorine

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(Masson, 1990). Reduced growth of microflora during subsequent storage was attributed to residualeffects of acetic acid released by degradation of peracetic acid. Peroxyacetic acid has been usedextensively as a sanitizer for food processing equipment where it is particularly effective againstbiofilms. It also holds potential as a disinfectant for fruits and vegetables, although its efficacy inkilling infectious microorganisms attached to the surface of plant materials has not been extensivelystudied.

The observation that washing or rinsing fruits and vegetables with organic acids will reducepopulations of some types of pathogenic bacteria would indicate that applying vinegar or lemonjuice holds promise as a simple and inexpensive household disinfection procedure. Indeed, suchtreatments are likely to reduce the risk of illness associated with potentially contaminated fruits andvegetables. A possible disadvantage is that these treatments may change the flavour and aroma oftreated products.

Hydrogen peroxide

Hydrogen peroxide (H2O2) can have a lethal or inhibitory effect on microorganisms, dependingon the pH, temperature and other environmental factors (Juven and Pierson, 1996). Saper (1996)has studied the efficacy of H2O2 in improving the microbiological quality and extending the shelf-life of minimally processed fruit and vegetable products. Hydrogen peroxide vapour treatmentswere highly effective in reducing microbial numbers on whole cantaloupes, grapes, prunes, raisins,walnuts and pistachios. However, similar treatment induced browning in mushrooms exposed tolevels necessary to delay spoilage by Pseudomonas tolaasii. Exposure to H2O2 vapour causedbleaching of anthocyanins in strawberries and raspberries. Dipping freshly-cut green bell pepper,cucumber, zucchini, cantaloupe and honeydew melon in H2O2 solution had no adverse effect onappearance, flavour or texture, but it induced severe browning of shredded lettuce. Dip treatmentsignificantly reduced the population of Pseudomonas on these products but had no measurableeffect on yeasts and moulds.

Reductions in populations of Salmonella on alfalfa sprouts are similar by dipping sprouts in 200and 500 ppm chlorine or 2% and 5% H2O2, respectively, for 2 minutes (Beuchat, 1997). Slightlymore than 2 log10 CFU/g reduction was observed after treatment with 200 ppm chlorine or 2%H2O2. The effectiveness of the same concentrations of chlorine and H2O2 in eliminating Salmonellafrom cantaloupe cubes was much less, however, with reductions of less than 1 log10 CFU/g. Alfalfasprouts and, to a lesser extent, cantaloupe cubes took on a mildly bleached appearance aftertreatment with 5% H2O2. Results from studies on a limited number of fruits and vegetables indicatethat H2O2 has potential for use as a disinfectant. Some fruits and vegetables (e.g. mushrooms, sometypes of berries and lettuce), however, may not be amenable to disinfecting with H2O2 because ofadverse changes in surface colour. Additional work to determine the effectiveness of H2O2 in killingpathogens on a wide range of raw fruits and vegetables is warranted. The occupational PEL in airin the United States is 1 ppm (8-hour time-weighted average) (OSHA).

Ozone

Treatment of drinking-water with ozone for the purpose of killing microorganisms has beenpractised for nearly a century. Salmonella Typhimurium, Y. enterocolitica, S. aureus, and L.monocytogenes are among the pathogens sensitive to treatment in ozonated (20 ppm) water(Restaino et al., 1995). Enteric viruses (Finch and Fairbairn, 1991) and oocysts of protozoa such as

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Cryptosporidium parvum (Korich et al., 1989) are also sensitive to ozone. Greater than 90%inactivation of C. parvum was achieved by treatment with 1 ppm ozone for 5 minutes (Peeters etal., 1990). C. parvum oocysts are about 30 times more resistant to ozone and 14 times moreresistant to ClO2 than Giardia cysts under the same conditions.

The use of ozone to disinfect various types of foods has been investigated. Preservation of fish(Haraguchi et al., 1969), reduction of aflatoxin in peanuts and cottonseed meals (Dwankanath etal., 1968), reduction of microbial populations on poultry (Sheldon and Brown, 1986), andreduction of microbial populations on bacon, beef, butter, cheese, eggs, mushrooms, potatoes andfruits (Gammon and Kerelak, 1973; Kaess and Weidemann, 1968) using gaseous ozone have beenstudied. High relative humidity or aqueous conditions generally favour microbicidal activity. Thelethal effect of ozone is a consequence of its strong oxidizing power. For this reason, physiologicalinjury of produce, for example in bananas, can result from exposure to concentrations as low as 1.5ppm without damage to sensory qualities (Horvath et al., 1985). After eight days, black spots appearon the skin of bananas if the ozone concentration is maintained at 25-30 ppm. Extension of shelf-life of oranges, strawberries, raspberries, grapes, apples and pears can be achieved by treatmentwith ozonated water. In addition to the antimicrobial effects of ozone, oxidation of ethylene alsooccurs, thus retarding metabolic processes associated with ripening. Concentrations in the range of2-3 ppm for berry fruits and 40 ppm for oranges greatly reduce microbial populations.

Because of its instability, ozone must be generated at the usage site. Users should also be awarethat, because of the strong oxidizing power of ozone, metal and other types of surfaces with whichit comes into contact are subject to corrosion or other deterioration. The rate of deteriorationdepends on the concentration of ozone. Nevertheless, the use of ozonated wash and flume-watersin fruit and vegetable handling and processing operations provides a method to control build-up ofmicrobial numbers, particularly in recycled water, and deserves consideration for routine use as adisinfectant for fruits and vegetables. The occupational PEL in air in the United States is 0.1 ppm (8-hour time-weighted average) (OSHA).

Irradiation

The application of ionizing irradiation (e.g. gamma rays from 60Co or 137Cs) to raw fruits andvegetables is a means of extending shelf-life (Thayer et al., 1996). Decay caused by indigenousmicroflora and post-handling contaminants can be eliminated or delayed by dose levels that do notadversely affect the sensory qualities of many fruits and vegetables. Dose levels of 1 to 3 kGy,depending upon the type of fruit or vegetable, are sufficient to kill large numbers of most moulds,yeasts and bacteria naturally present on produce as it is taken from the field (Farkas, 1997; Moy,1983; Urbain, 1986).

Factors influencing lethality include the level of sensitivity of the target insect ormicroorganism(s) to irradiation, post-harvest and pretreatment conditions of fruits and vegetables,and environmental conditions (e.g. relative humidity and temperature) surrounding fruits andvegetables at the time of treatment. Extensive studies to determine the efficacy of irradiationtreatment in disinfestation (CAST, 1986; 1989) and abating post-harvest fungal diseases have beenreviewed (Clarke, 1959; Willison, 1963; Moy, 1983; Wilkinson and Gould, 1996). Botrytis,Rhizopus and Mucor species are among the major spoilage moulds of raw fruits and vegetables,and are inactivated by dose levels of 1.2 to 3.0 kGy. A dose of 3 kGy, for example, will extend theshelf-life of strawberries stored at 5°C by about 1 week (Sommer and Maxie, 1966). Guidelines fordisinfestation and quarantine treatment of raw fruits and vegetables have been issued (ICGFI,1991a; 1991b; ASTM, 1993).

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The effectiveness of ionizing irradiation in killing microorganisms capable of causing humanillness has been studied but most investigations have been done using in vitro conditions or foodsof animal origin (Farkas, 1989; 1997; Monk et al., 1995; Mossel and Stegeman, 1985). Vegetativecells of pathogenic bacteria are sensitive to irradiation, D10 values being in the range of 0.2-0.8 kGy,depending on environmental conditions. Foodborne parasites are also generally sensitive toirradiation. With the exception of some nematodes such as Aniostrongylus cantonensis,Gnatostoma spinigerum and Anisakis sp., a dose below 1 kGy is sufficient to control infectioncaused by protozoa, trematodes and cestodes (Loaharanu and Murrell 1994). Higher doses arerequired, however, for inactivation of viruses. D10 values for inactivating viruses (Mallett et al.,1991; Sullivan et al., 1973) may be more than 10-fold higher. Therefore, irradiation cannot be usedfor inactivation of viruses. Nevertheless, ionizing irradiation could be an extremely effective tool inreducing populations of pathogenic microorganisms and parasites from the surface of raw fruits andvegetables. Its application on a large scale for the purpose of reducing the risk of disease associatedwith the consumption of potentially contaminated raw fruits and vegetables would appear to haveexceptional merit. There is a need, however, to evaluate the tolerance of most fruits and vegetablesto the radiation doses required for controlling various pathogenic microorganisms.

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26 BLANCHE

27


Conclusion

Several pathogenic bacteria, viruses and parasites capable of causing human disease can be foundon raw fruits and vegetables. Some of these microorganisms are capable of growing on whole,minimally processed or cut fruits and vegetables under routine handling and storage conditions. Itis essential that interventions be made to prevent contamination of raw fruits and vegetables and,failing this, to remove disease-causing microorganisms prior to consumption. However, none of thechemical or physical treatments currently used to disinfect raw fruits and vegetables can be reliedon to eliminate all types of pathogens from the surface or internal tissues when using applicationconditions that will not adversely affect sensory or nutritional qualities. Irradiation is a powerful toolagainst bacteria and parasites but cannot be relied on to eliminate viruses.

The efficacy of various disinfectants and sanitizing methods for reducing populations ofmicroorganisms on raw fruits and vegetables varies greatly. Differences in surface characteristicsof fruits and vegetables, type and physiological state of microbial cells, and environmental stressconditions interact to influence the activity of disinfectants and sanitizers. Conditions under whichchlorine is effective in killing microorganisms on fruits and vegetables have been studied mostextensively. Nevertheless, its performance in relation to other chemical and physical treatmentsunder various conditions of application has not been clearly defined. Although there is a lack ofextensive scientific data from which to extract firm conclusions concerning the efficacy ofdisinfectants for raw fruits and vegetables, some general conclusions can be drawn:

l Efficacy of disinfectants varies with different fruits and vegetables, characteristics of theirsurfaces, temperature and type of pathogen.

l Listeria monocytogenes is generally more resistant to disinfectants than Salmonella,Escherichia coli O157:H7 and Shigella.

l Little is known about the efficacy of disinfectants in killing parasites and viruses on fruits andvegetables.

l Washing fruits and vegetables in potable water removes a portion of microbial cells. In someinstances, vigorous washing can be as effective as treatment with water containing 200ppm chlorine, which generally reduces populations by 10-100-fold.

l Heavily contaminated fruits and vegetables should be subjected to a double wash treatment.Success in removing soil or faecal matter, and the contaminants therein, is more likely to beachieved by first washing in potable water and then washing or rinsing in water containinga disinfectant.

l The temperature of wash-water should be higher than that of the fruits or vegetables in orderto minimize uptake of microorganisms by tissues.

l The lethal effect of chlorine occurs within the first few seconds of treatment. The populationof microorganisms decreases as the concentration of chlorine increases to about 300 ppm,above which effectiveness is not proportional to increased concentration.

l Leaving fruits and vegetables wet after disinfecting or washing can negate any beneficialeffect of treatment.

l Chlorine dioxide is useful in controlling populations of microorganisms in wash-water butvaries in efficacy in killing microorganisms on the surface of fruits and vegetables.

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l Bromine and iodine may have limited potential as disinfectants for fruits and vegetables,partly because of their adverse effect on sensory quality.

l Trisodium phosphate has good potential as a disinfectant for whole fruits and vegetables ina commercial setting. Use in households may be limited, however, because the highalkalinity of TSP may cause skin irritation.

l Although disinfectants have variable effects on pathogen control on fresh fruits andvegetables, they are certainly useful for sanitizing wash-water to prevent contamination ofthe produce that could result from using waters that are not microbiologically safe.

l Organic acids (e.g. acetic, lactic, citric and peroxyacetic acids) have good potential asdisinfectants for fruits and vegetables, but conditions under which they are most effectivehave not been defined.

l Ozonation of wash-water reduces numbers of microorganisms, thus resulting in reducednumbers on the surfaces of fruits and vegetables.

l Faecal matter or water containing faeces should never come into contact with fruits andvegetables, as even the most powerful treatment (e.g. irradiation) cannot be relied on toeliminate some of the pathogens they may contain.

l Prevention of contamination of fruits and vegetables with pathogens at all points from thefield to the plate, through application of good agricultural practices (GAP), goodmanufacturing practices (GMP) and HACCP programmes, is preferred to application ofchemical disinfectants after contamination has occurred.

There is a lack of information on the extent and type of microbial contamination of raw fruits andvegetables on an international scale. Nevertheless, observations support these conclusions, andcan serve as a basis for developing recommendations and plans of action.

Future directions and recommendations

Application of the principles inherent in or deriving from the following recommendations will resultin reduced risk of illness associated with raw fruits and vegetables.

1. Further basic and applied research is needed in order:- to better understand modes of contamination of raw fruits and vegetables before harvesting,

when subjected to various agronomic practices, and during post-harvest handling;- to determine conditions that influence attachment, growth, survival and death of human

pathogenic microorganisms on fruits and vegetables;- to establish and validate disinfection methods, individually and in combination, that

minimize risk of illness caused by microorganisms on fruits, vegetables and other plantmaterials eaten raw (such research should also address the short-term and long-term toxicityof residues or reaction products resulting from chemical or physical treatments);

- to develop highly effective treatments for removing pathogens from a wide range of rawproduce (a single type of treatment will not be suitable for all fruits and vegetables. Cautionmust be taken in the development of sanitizing or disinfecting treatments so that degradationof nutritional components of fruits and vegetables is minimal and the formation ofcompounds with potential toxic or carcinogenic properties is avoided);

- to develop or improve analytical techniques to detect low numbers of microorganisms,particularly viruses and parasites, on or in raw fruits, vegetables and plant materials beforeand after treatment with various disinfectants.

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2. Control of pathogenic microorganisms should involve multidisciplinary teams with a widerange of technical, sociological, educational and administrative skills. Health educationprogrammes should fully describe the impact of infections that can be associated with raw fruits andvegetables.

3. Hygienic principles should be applied from production to consumption of fruits and vegetables(agriculture, transport, manufacturing and processing, distribution, and preparation forconsumption). Disinfection should be applied where appropriate. However, interventions tominimize contamination of fruits and vegetables at any point in the chain should be preferred toremedial or corrective action.

4. Effective surveillance programmes should be established to determine the presence andprevalence of specific pathogenic bacteria, viruses and parasites on raw fruits and vegetables at thepoint of importation.

5. Epidemiological studies should be carried out to address the role of raw fruits and vegetablesas vehicles of microorganisms capable of causing human diseases.

6. There should be continual training of fruit and vegetable growers and handlers at all levels inorder to control microbiological hazards that may be influenced by current and changingaquacultural, agronomic, processing, distribution and preparation practices.

7. Conferences, workshops and symposia specifically addressing the control of human infectionsassociated with the consumption of contaminated raw fruits and vegetables should be held inplaces where they will have greatest impact.

8. Professional and domestic food handlers and consumers should be better educated about theprinciples of personal hygiene and decontamination of raw fruits, vegetables and plant materials.Changes in risk of illness related to ageing and health status of consumers should be more clearlydefined.

9. Quantitative microbiological risk assessment of various human infections and intoxicationsthat can be linked to the consumption of contaminated raw fruits, vegetables and plant materialsshould be undertaken.

10. Cost-benefit and environmental impact assessments should be made for each method ofdisinfection.

11. The establishment of effective GAP, GMP and HACCP programmes that would cover allaspects of growing, harvesting, packing, transport, processing, distribution, and preparation of rawfruits and vegetables is strongly encouraged. Since contamination of raw produce with pathogenscan occur at any point in the system, all stages must be considered when devising HACCPprogrammes. Assistance from, and collaboration between, academic institutions, public healthauthorities, food control agencies, trade organizations and the private sector in developing HACCPprogrammes for the raw fruit and vegetable industry will be necessary to minimize the risk ofdisease. An integral part of this process will involve identifying critical control points at whichmeasures should be taken to prevent contamination and to decontaminate. In the interim, wherelack of science-based information prevents identification of critical control points for specificsegments of the fruit or vegetable growing and handling chain, this should be acknowledged andGAP and GMP established. The development of HACCP programmes will eventually follow.

12. Several diseases caused by viruses, parasites and some types of pathogenic bacteria that arelinked to the consumption of raw fruits and vegetables are transmitted via the faecal-oral route. It

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is imperative that individuals handling raw produce at every stage, from the field to the point ofconsumption, have an understanding of hygiene sufficient to prevent contamination. Applicationof improperly composted manure or water containing raw sewage to fields, or the use of watercontaminated with faeces during processing of fruits and vegetables, must be avoided. Theseresponsibilities must be shared by the grower as well as the processor, shipper, distributors and foodhandlers. Training of food handlers at all level of the food chain, as well as education of theconsumer, are key elements in a total system approach to reducing the risk of produce-borneillnesses.

13. Finally, but not least in importance, there is a need to further develop and enhanceinternational epidemiological surveillance programmes for foodborne diseases. Collaboration insuch programmes, whether between countries or continents, will generate information that will beuseful in developing guidelines for reducing the risk of disease that may be caused by raw fruits andvegetables.

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FOOD SAFETY UNITWORLD HEALTH ORGANIZATION

Surface decontaminationof

fruits and vegetableseaten raw:a review

Food Safety Issues

WHO/FSF/FOS/98.2Original English

Distr. General

44

WHO/FSF/FOS/98.2

Surface decontamination offruits and vegetables eaten raw: a review

prepared by

Larry R. Beuchat, Ph.D.Center for Food Safety and Quality Enhancement

University of Georgia, Griffin, Georgia, USA

in collaboration with

NSF InternationalWHO Collaborating Centre for Drinking Water Safety and Treatment

Washington, D.C., USAWHO Collaborating Centre for Food Safety

Brussels, Belgium

WHO Collaborating Centre for Foodborne Disease SurveillanceCenters for Disease Control and Prevention (CDC)

Atlanta, Georgia, USA

National Institute of Public Health and Environmental ProtectionWHO Collaborating Centre for Food Safety

Bilthoven, The Netherlands

WHO/FSF/FOS/98.2Original English

Distr. General

45


Contents

PagePreface ......................................................................................................................... ii

Acknowledgements ..................................................................................................... iii

Summary ..................................................................................................................... iv

Introduction .................................................................................................................1

Recognizing the problem .............................................................................................3

Pathogens of most concern ..........................................................................................9Salmonella ..............................................................................................................9Shigella ...................................................................................................................9Escherichia coli .....................................................................................................10Campylobacter ......................................................................................................10Yersinia enterocolitica ...........................................................................................11Listeria monocytogenes .........................................................................................11Staphylococcus aureus ..........................................................................................11Clostridium species ...............................................................................................12Bacillus cereus ......................................................................................................12Vibrio species .......................................................................................................12Viruses ..................................................................................................................13Parasites ................................................................................................................13

Mitigating the risk of disease ......................................................................................15Chlorine ................................................................................................................16Chlorine dioxide ...................................................................................................19Bromine ................................................................................................................20Iodine ...................................................................................................................20Trisodium phosphate .............................................................................................21Quaternary ammonium compounds ......................................................................21Acids.....................................................................................................................21Hydrogen peroxide ...............................................................................................23Ozone...................................................................................................................23Irradiation .............................................................................................................24

Conclusion .................................................................................................................27Future directions and recommendations .....................................................................28References .................................................................................................................31

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WHO/FSF/FOS/98.2

Preface

Prevention of contamination is the most efficient way to ensure food safety andprevent foodborne illness. Thus, every effort should be made to protect food from

primary sources of contamination. This is, however, not always possible. Raw foodstuffs,particularly fruits and vegetables grown close to the soil, may be contaminated with variousfoodborne pathogens.

In some countries, chemical disinfectant agents are used to decontaminate the surface of fruitsand vegetables, in addition to washing with water. It is not clearly known to which extent theseagents are effective, what their optimum conditions of use are, and whether they have adversetoxicological effects.

The present document attempts to provide an insight into some of these concerns. It presents areview of pathogens associated with fruits and vegetables and a literature review of studies carriedout to investigate the efficacy of a range of methods used for surface decontamination. However,it does not review the potential adverse effects of using chemical disinfectants. Further work isneeded on this subject. Therefore, the document should not be seen as a recommendation of theuse of chemical disinfectant agents for surface decontamination of fruits and vegetables.

iiii

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iii

Acknowledgements

FAO and WHO would like to acknowledge with thanks the contribution of the followingpersons in reviewing the document:

Simin Abrishami, National Sanitation Foundation (NSF) International, USAMartin R. Adams, University of Surrey, United KingdomJudit Beczner, KEKI (Central Food Research Institute), HungaryBob Buchanan, US Department of Agriculture, USAJorge Chirife, Ciudad Universitaria, ArgentinaDean O. Cliver, University of California, USAJoseph A. Cotruvo,National Sanitation Foundation (NSF) International, USAJohn Crowther, Colworth Laboratory, United KingdomCaj Eriksson, IUFoST, SwedenJeffery Farber, Health Canada, CanadaOwen Fennema, University of Wisconsin-Madison, USADilma Scala Gelli, Instituto Adolfo Lutz, BrazilArie H. Havelaar, RIVM (National Institute of Public Health & the Environment,

NetherlandsRobert E. Harrington, National Restaurant Association, USAMichael Heinzel, Henkel KGaA, GermanyGeorge Jackson, Food and Drug Administration, USAPaisan Loaharanu, International Atomic Energy Agency, AustriaBert van Klingeren, RIVM (National Institute of Public Health & the Environment),

NetherlandsTai Wan Kwon, INJE University, KoreaMorris Potter, Centre for Disease Control, USAS. Kay Puryear, Proctor & Gamble, USAWill Robertson, Health Canada, CanadaJocelyn Rocourt, Institut Pasteur, FranceMichiel van Schothorst, International Commission on Microbiological Specifications

for Food, SwitzerlandDavid Statham, Chartered Institute of Environmental Health, United KingdomEghbal Taheri, Ministry of Health and Medical Education, IranGerald Townsend, HJ Heinz, Ltd., New ZealandRay J. Winger, Massey University, New ZealandStacey A. Zawei, United Fresh Fruit and Vegetables, USASteve A. Ziller, Jr., Grocery Manufacturers of America, Inc., USA

FAO and WHO would like to thank the International Life Sciences Institute (ILSI) and the InternationalUnion of Food Science and Technology (IUFoST) for having assisted in the review of the document.Special thanks is also conveyed to the Japanese Government for having supported this projectfinancially.

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WHO/FSF/FOS/98.2

Summary

Outbreaks of human diseases associated with the consumption of raw fruits and vegetablesoften occur in developing countries and have become more frequent in developedcountries over the past decade. Factors thought to influence the occurrence and

epidemiology of these diseases include irrigation and other agronomic practices, the general levelof hygiene in handling fruits and vegetables, international travel, globalization of the supply anddistribution of raw produce, the introduction of pathogens into new geographical areas, changes inthe virulence and environmental resistance of pathogens, decrease in immunity among certainparts of the population (particularly the elderly), and changes in eating habits.

Fruits and vegetables eaten raw, as well as food of animal origin, have long been known to serveas vehicles for transmission of infectious microorganisms in developing countries. The number ofconfirmed cases of illness associated with raw fruits and vegetables in industrialized countries, onthe other hand, has been relatively low compared to the number of recorded cases due to foods ofanimal origin. The impact of agronomic practices, handling, processing, distribution and marketingon risks associated with microbiological hazards of raw fruits and vegetables is not wellunderstood.

This document reviews the health hazards associated with fruits and vegetables and the effectsof different decontamination methods in eliminating or reducing the microbial load. Disinfectantsare effective in reducing the bacterial load but their efficacy depends on the types of fruits andvegetables and characteristics of their surface, the method and the procedure used for disinfection(e.g. temperature, pH etc), and on the type of pathogen. Listeria monocytogenes is generally moreresistant to disinfectants than Salmonella, pathogenic Escherichia coli and Shigella, but little isknown about the efficacy of disinfectants in killing parasites and viruses on fruits and vegetables.Vigorously washing fruits and vegetables with safe water reduces the number of microorganisms by10-100-fold and is often as effective as treatment with 200 ppm chlorine. Treatments with chlorinedioxide, trisodium phosphate, organic acids or ozone have potential for removing pathogenicmicroorganisms from raw fruits and vegetables. When applicable, food irradiation is one of themost powerful methods of decontamination.

Prevention of contamination at all points of the food chain from primary production to the finalconsumer is preferred over the application of disinfectant after contamination occurs. Preventioncan be achieved through application of the principles of food hygiene and the Hazard Analysis andCritical Control Point system (HACCP).

Research is needed to better understand not only the mechanisms through which pathogens cancontaminate raw fruits and vegetables but also the procedures for killing or removing pathogensonce they are present, either on the surface or in internal tissues, and the analytical methods for theirdetection. A quantitative microbiological risk assessment should be undertaken of humaninfections and intoxications that can be linked to the consumption of contaminated raw fruits,vegetables and plant materials. The development of a highly efficient, internationalepidemiological surveillance system for better understanding the role of raw fruits and vegetablesas vehicles for infectious diseases is critical. Information generated by such a system would bevaluable in establishing more meaningful practices and guidelines for preventing contaminationand for decontaminating raw fruits and vegetables.

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Continual training of fruit and vegetable growers and handlers at all levels should be carried outin order to control microbiological hazards that may be influenced by current and changingpractices in aquaculture, agronomy, processing, marketing and preparation. The use of water andfertilizers free of pathogenic microorganisms at all stages of growing and handling and, just asimportantly, an appropriate level of understanding of the importance of personal hygiene byeveryone involved is essential if the risk of illness is to be minimized. Regardless of the stringencyof hygienic measures at production and processing stages of the food chain, consumers should beeducated to realize the importance of adequate washing of fruits and vegetables.

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Surface Decontamination of Fruits and Vegetables Eaten Raw

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